Cable with improved electrical properties

ABSTRACT

A cable comprising a conductor surrounded by at least an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order, wherein the insulation layer is not crosslinked or is cross-linked and comprises at least 90 wt % of a polymer composition, said polymer composition comprising: (I) 95.5 to 99.9 wt % of an LDPE; and (II) 0.1 to 4.5 wt % of an HDPE having a density of at least 940 kg/m3.

This invention relates to the use of blends comprising low densitypolyethylene (LDPE) polymers in the insulation layers of cables, such asDC cables. In particular, the invention relates to the combination ofLDPEs with low amounts of a secondary HDPE polymer in order tosurprisingly enable the formation of cable insulation layers withremarkably low conductivity.

The blends of the invention can be used in cross-linked ornon-cross-linked form, in the latter case avoiding the need for acrosslinking agent to be present and avoiding the need for a postcrosslinking degassing procedure to remove crosslinking agentby-products.

Whether cross-linked or non-cross-linked, the conductivity of theresulting composition and hence the conductivity of the insulation layeris lower than the use of the corresponding LDPE alone despite very lowlevels of HDPE being employed.

BACKGROUND

Polyolefins produced in a high pressure (HP) process are widely used indemanding polymer applications where the polymers must meet highmechanical and/or electrical requirements. For instance in power cableapplications, particularly in medium voltage (MV) and especially in highvoltage (HV) and extra high voltage (EHV) cable applications, theelectrical properties of the polymer composition used in the cable hassignificant importance. Furthermore, the electrical properties ofimportance may differ in different cable applications, as is the casebetween alternating current (AC) and direct current (DC) cableapplications.

A typical power cable comprises a conductor surrounded, at least, by aninner semiconductive layer, an insulation layer and an outersemiconductive layer, in that order. The cables are commonly produced byextruding the layers on a conductor.

The polymer material in one or more of said layers is often cross-linkedto improve e.g. heat and deformation resistance, creep properties,mechanical strength, chemical resistance and abrasion resistance. Duringthe crosslinking reaction, crosslinks (bridges) are primarily formed.Crosslinking can be effected using e.g. a free radical generatingcompound which are typically incorporated into the layer material priorto the extrusion of the layer(s) on a conductor. After formation of thelayered cable, the cable is then subjected to a crosslinking step toinitiate the radical formation and thereby crosslinking reaction.

Peroxides are very commonly used as free radical generating compounds.The resulting decomposition products of peroxides may include volatileby-products which are often undesired, since e.g. may have a negativeinfluence on the electrical properties of the cable. Therefore thevolatile decomposition products such as methane are conventionallyreduced to a minimum or removed after crosslinking and cooling step.Such removal step, generally known as a degassing step, is time andenergy consuming causing extra costs. It will be appreciated that across-linked polyethylene material is thermosetting.

LDPE is also an ideal cable forming material from a cleanliness point ofview. LDPE can be manufactured in very pure form without impurities. Incontrast low pressure polymers often contain more gels and catalystresidues which can lead to defects in the cable.

In order to increase the power transmission capability of extruded highvoltage direct current (HVDC) cables, the voltage needs to be increased.In HVDC cables, the insulation is heated by the leakage current. Theheating is proportional to the insulation conductivity×voltage². Thus,if the voltage is increased, more heat will be generated. This may leadto thermal runaway followed by electric breakdown. Thus, in order toincrease the power transmission capacity, insulation material with verylow electrical conductivity is needed. In one embodiment, the voltagemay be increased from today's highest level of 320 kV to 640 kV or more.

The present inventors have now investigated the possibility of reducingconductivity though combination of the LDPE with a secondary polymer.The secondary polymer however is typically one made using an olefinpolymerisation catalyst and hence catalyst residue content might behigh. This leads to a greater risk of mechanical breakdown compared topure XLPE. Nevertheless, the inventors have surprisingly found that thecombination of low amounts of HDPE and LDPE leads to remarkableconductivity reduction in thermoplastic and cross-linked insulationlayers even at very low levels of HDPE.

Thermoplastic LDPE offers several advantages as cable insulationcompared to a thermosetting cross-linked PE. As the polymer is notcross-linked, there is no possibility of peroxide initiated scorch. Inaddition, no degassing step is required to remove peroxide decompositionproducts. The elimination of crosslinking and degassing steps can leadto faster, less complicated and more cost effective cable production.However, the absence of a cross-linked material can lead to a reducedtemperature resistance and hence significant problems with creep. Thus,better thermomechanical properties are needed in order to provide apolymer material that can be used without crosslinking in a cableinsulation layer.

The present inventors have now found that the combination of an LDPEwith a low amount of an HDPE can provide a blend such as a thermoplasticblend which is ideally suited for cable manufacture. Surprisingly, theseblends have much lower conductivity than the corresponding LDPE aloneand do not suffer from dielectric breakdown. Moreover, as the HDPEcontent is so low, this leads to a reduced risk of mechanical breakdowncaused by the presence of the less pure HDPE with the LDPE.

Moreover, the inventors have found that certain LDPEs can be combinedwith low amounts of HDPE to form a blend which has remarkably lowconductivity in cross-linked form.

The LDPE of use in the invention is not itself new and it has beenpreviously proposed in the literature. Moreover, the possibility ofusing non cross-linked LDPE in the insulation layer of a cable is notnew. In WO2011/113685, LDPE of density 922 kg/m³ and MFR₂ 1.90 g/10 minis suggested for use in the insulation layer of a cable. WO2011/113685also suggests using other polymers individually in the non cross-linkedinsulation layer of a cable.

In WO2011/113686, a blend of LDPE and HDPE is used to manufacture across-linked polymer composition that can be used in the insulationlayer of a cable however the amounts of HDPE taught are relatively high(minimum 5 wt %).

In US2013/0175068 there is a disclosure of the use of HDPE and LDPE toimprove breakdown strength in thermoplastic cables. 20 wt % HDPE isexemplified in the examples.

The blends of the invention are therefore ideal for use in theinsulation layer in a direct current (DC) power cable or AC power cableand the blends enable cables that operate at voltages higher thanpossible today.

SUMMARY OF INVENTION

Viewed from one aspect the invention provides a cable comprising one ormore conductors surrounded by at least an inner semiconductive layer, aninsulation layer and an outer semiconductive layer, in that order,wherein the insulation layer is not cross-linked or is cross-linked andcomprises at least 90 wt % of a polymer composition, said polymercomposition comprising:

-   -   (I) 95.5 to 99.9 wt % of an LDPE; and    -   (II) 0.1 to 4.5 wt % of an HDPE having a density of at least 940        kg/m³.

Viewed from one aspect the invention provides a cable comprising one ormore conductors surrounded by at least an inner semiconductive layer, aninsulation layer and an outer semiconductive layer, in that order,wherein the insulation layer is cross-linked and comprises at least 90wt % of a polymer composition, said polymer composition comprising:

-   -   (I) 95.5 to 99.9 wt % of an LDPE; and    -   (II) 0.1 to 4.5 wt % of an HDPE having a density of at least 940        kg/m³.

Viewed from another aspect the invention provides a cable comprising oneor more conductors surrounded by at least an inner semiconductive layer,an insulation layer and an outer semiconductive layer, in that order,wherein the insulation layer is not cross-linked and comprises at least90 wt % of a polymer composition, said polymer composition comprising:

-   -   (I) 95.5 to 99.9 wt % of an LDPE; and    -   (II) 0.1 to 4.5 wt % of an HDPE, preferably a multimodal HDPE,        having a density of at least 940 kg/m³.

Where the insulation layer is not cross-linked, the layer should be freeof any crosslinking agent such as a peroxide.

In one embodiment the HDPE component (II) is bimodal, especially wherethe insulation layer is not crosslinked.

In particular the cable of the invention is a direct current (DC) powercable, preferably operating at or capable of operating at 320 kV or moresuch as 650 kV or more.

Viewed from another aspect the invention provides a process forproducing a cable comprising the steps of:

-   -   applying on one or more conductors, preferably by (co)extrusion,        an inner semiconductive layer, an insulation layer and an outer        semiconductive layer, in that order, wherein the insulation        layer comprises at least 90 wt % of a polymer composition as        herein before defined and optionally cross-linking the layer.

Viewed from another aspect the invention provides the use of acomposition as hereinbefore defined in the insulation layer of a cable.

In all embodiments of the invention it is preferred if the polymercomposition of the insulation layer has a conductivity of 1.5 fS/m orless when measured according to DC conductivity method as describedunder “Determination Method A”.

In all embodiment it is preferred if the conductivity of the polymercomposition of the insulation layer is 3.5×10⁻¹⁷ S/cm or less whenmeasured according to DC conductivity method as described under“Determination Methods B”.

Definitions

Non cross-linked polymer compositions or cable layers are regarded asthermoplastic.

The polymer composition of the invention may also be referred to as apolymer blend herein. These terms are used interchangeably.

The low density polyethylene, LDPE, of the invention is a polyethyleneproduced in a high pressure process. Typically the polymerization ofethylene and optional further comonomer(s) in a high pressure process iscarried out in the presence of an initiator(s). The meaning of the termLDPE is well known and documented in the literature. The term LDPEdescribes and distinguishes a high pressure polyethylene frompolyethylenes produced in the presence of an olefin polymerisationcatalyst. LDPEs have certain typical features, such as differentbranching architecture.

DETAILED DESCRIPTION OF INVENTION

The present invention requires the use of a particular polymercomposition comprising low density polyethylene (LDPE) and low amountsof HDPE in the insulation layer of a cable, especially a power cablesuch as a direct current (DC) power cable. Unexpectedly, the combinationof low amounts of HDPE with the LDPE enables the formation of aninsulation layer that has reduced, i.e. low, electrical conductivity.“Reduced” or “low” electrical conductivity as used hereininterchangeably means that the value obtained from the DC conductivitymeasurement as defined below under “Determination methods” is low, i.e.reduced. Low electrical conductivity is beneficial for minimisingundesired heat formation in the insulation layer of a cable.

Moreover and unexpectedly, some of the LDPE compositions of theinvention and hence the insulation layer of the cable of the inventionhave low electrical conductivity without the need for crosslinking.Furthermore, the insulation still possesses the mechanical propertiesneeded for an insulation layer of a cable, preferably a DC power cable,such as low flex modulus, good tensile modulus, and good stress crack.

It is also remarkable that such low levels of conductivity areachievable at such low levels of HDPE addition. Because the amount ofHDPE added is so low, the blend is “cleaner”. LDPE can be manufacturedin very pure form without impurities but as soon as LDPE is blended witha low pressure polymer such as HDPE, more gels and catalyst residues areintroduced which can lead to defects in the composition. It is thereforepreferred if the addition of the HDPE is kept to a minimum to maximisethe purity of the insulation layer. It was perceived however that lowlevels of HDPE addition would not lead to marked improvements inconductivity.

Remarkably, we observe that at very low levels of HDPE addition,conductivity is comparable or sometimes better (i.e. lower) thanconductivity achieved at higher HDPE loading. Hence we can minimiseconductivity and prepare a purer, defect free insulation layer.

LDPE

The low density polyethylene, LDPE, of the invention is a polyethyleneproduced in a high pressure process. Typically the polymerization ofethylene and optional further comonomer(s) in a high pressure process iscarried out in the presence of an initiator(s). The meaning of the termLDPE is well known and documented in the literature. The term LDPEdescribes and distinguishes a high pressure polyethylene frompolyethylenes produced in the presence of an olefin polymerisationcatalyst. LDPEs have certain typical features, such as differentbranching architecture.

LDPE Homopolymer or Copolymer

The LDPE used in the composition of the invention may have a density of915 to 940 kg/m³, preferably 918 to 935 kg/m³, especially 920 to 932kg/m³, such as about 922 to 930 kg/m³.

The LDPE polymer of the invention may be one having a high density. Thedensity of LDPE polymer is preferably 927 to 940 kg/m³, preferably 928to 935 kg/m³, especially 929 to 932 kg/m³, such as about 930 kg/m³. Inparticular, this higher density range is employable with thecross-linked multimodal HDPE polymer.

The MFR₂ (2.16 kg, 190° C.) of the LDPE polymer is preferably from 0.05to 30.0 g/10 min, more preferably is from 0.1 to 20 g/10 min, and mostpreferably is from 0.1 to 10 g/10 min, especially 0.1 to 5.0 g/10 min.In a preferred embodiment, the MFR₂ of the LDPE is 0.1 to 4.0 g/10 min,especially 0.5 to 4.0 g/10 min, especially 1.0 to 3.0 g/10 min.

The LDPE may have a tensile modulus (1 mm/min ISO527-2) of at least 300MPa, such as at least 325 MPa. Values up to 600 MPa are possible.

The LDPE may have a flex modulus (ISO178) of at least 300 MPa, such asat least 320 MPa. Values up to 600 MPa are possible.

It is possible to use a mixture of LDPEs in the polymer composition ofthe invention however it is preferred if a single LDPE is used.

The LDPE may be a low density homopolymer of ethylene (referred hereinas LDPE homopolymer) or a low density copolymer of ethylene with one ormore comonomer(s) (referred herein as LDPE copolymer). The one or morecomonomers of the LDPE copolymer are preferably selected from the polarcomonomer(s), non-polar comonomer(s) or from a mixture of the polarcomonomer(s) and non-polar comonomer(s). Moreover, said LDPE homopolymeror LDPE copolymer may optionally be unsaturated.

As a polar comonomer for the LDPE copolymer, comonomer(s) containinghydroxyl group(s), alkoxy group(s), carbonyl group(s), carboxylgroup(s), ether group(s) or ester group(s), or a mixture thereof, can beused. More preferably, comonomer(s) containing carboxyl and/or estergroup(s) are used as said polar comonomer. Still more preferably, thepolar comonomer(s) of LDPE copolymer is selected from the groups ofacrylate(s), methacrylate(s) or acetate(s), or any mixtures thereof.

If present in said LDPE copolymer, the polar comonomer(s) is preferablyselected from the group of alkyl acrylates, alkyl methacrylates or vinylacetate, or a mixture thereof. Further preferably, said polar comonomersare selected from C₁- to C₆-alkyl acrylates, C₁- to C₆-alkylmethacrylates or vinyl acetate. Still more preferably, said LDPEcopolymer is a copolymer of ethylene with C₁- to C₄-alkyl acrylate, suchas methyl, ethyl, propyl or butyl acrylate, or vinyl acetate, or anymixture thereof.

As the non-polar comonomer(s) for the LDPE copolymer, comonomer(s) otherthan the above defined polar comonomers can be used. Preferably, thenon-polar comonomers are other than comonomer(s) containing hydroxylgroup(s), alkoxy group(s), carbonyl group(s), carboxyl group(s), ethergroup(s) or ester group(s). One group of preferable non-polarcomonomer(s) comprise, preferably consist of, monounsaturated (=onedouble bond) comonomer(s), preferably olefins, preferably alpha-olefins,more preferably C₃ to C₁₀ alpha-olefins, such as propylene, 1-butene,1-hexene, 4-methyl-1-pentene, styrene, 1-octene, 1-nonene;polyunsaturated (=more than one double bond) comonomer(s); a silanegroup containing comonomer(s); or any mixtures thereof. Thepolyunsaturated comonomer(s) are further described below in relation tounsaturated LDPE copolymers.

If the LDPE polymer is a copolymer, it preferably comprises 0.001 to 35wt.-%, still more preferably less than 30 wt.-%, more preferably lessthan 25 wt.-%, of one or more comonomer(s). Preferred ranges include 0.5to 10 wt %, such as 0.5 to 5 wt % comonomer.

The LDPE polymer, may optionally be unsaturated, i.e. may comprisecarbon-carbon double bonds (—C═C—). Preferred “unsaturated” LDPEscontains carbon-carbon double bonds/1000 carbon atoms in a total amountof at least 0.4/1000 carbon atoms. If a non-cross-linked LDPE is used inthe final cable, then the LDPE is typically not unsaturated as definedabove. By not unsaturated is meant that the C═C content is preferablyless than 0.2/1000 carbon atoms, such as 0.1/1000 C atoms or less.

As well known, the unsaturation can be provided to the LDPE polymer bymeans of the comonomers, a low molecular weight (Mw) additive compound,such as a crosslinking booster, CTA or scorch retarder additive, or anycombinations thereof. The total amount of double bonds means hereindouble bonds added by any means. If two or more above sources of doublebonds are chosen to be used for providing the unsaturation, then thetotal amount of double bonds in the LDPE polymer means the sum of thedouble bonds present. Any double bond measurements are carried out priorto optional crosslinking.

The term “total amount of carbon-carbon double bonds” refers to thecombined amount of double bonds which originate from vinyl groups,vinylidene groups and trans-vinylene groups, if present.

If an LDPE homopolymer is unsaturated, then the unsaturation can beprovided e.g. by a chain transfer agent (CTA), such as propylene, and/orby polymerization conditions. If an LDPE copolymer is unsaturated, thenthe unsaturation can be provided by one or more of the following means:by a chain transfer agent (CTA), by one or more polyunsaturatedcomonomer(s) or by polymerisation conditions. It is well known thatselected polymerisation conditions such as peak temperatures andpressure, can have an influence on the unsaturation level. In case of anunsaturated LDPE copolymer, it is preferably an unsaturated LDPEcopolymer of ethylene with at least one polyunsaturated comonomer, andoptionally with other comonomer(s), such as polar comonomer(s) which ispreferably selected from acrylate or acetate comonomer(s). Morepreferably an unsaturated LDPE copolymer is an unsaturated LDPEcopolymer of ethylene with at least polyunsaturated comonomer(s).

The polyunsaturated comonomers suitable for the unsaturated secondpolyolefin (b) preferably consist of a straight carbon chain with atleast 8 carbon atoms and at least 4 carbons between the non-conjugateddouble bonds, of which at least one is terminal, more preferably, saidpolyunsaturated comonomer is a diene, preferably a diene which comprisesat least eight carbon atoms, the first carbon-carbon double bond beingterminal and the second carbon-carbon double bond being non-conjugatedto the first one. Preferred dienes are selected from C₈ to C₁₄non-conjugated dienes or mixtures thereof, more preferably selected from1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene,7-methyl-1,6-octadiene, 9-methyl-1,8-decadiene, or mixtures thereof.Even more preferably, the diene is selected from 1,7-octadiene,1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene, or any mixturethereof, however, without limiting to above dienes.

It is well known that e.g. propylene can be used as a comonomer or as achain transfer agent (CTA), or both, whereby it can contribute to thetotal amount of the carbon-carbon double bonds, preferably to the totalamount of the vinyl groups. Herein, when a compound which can also actas comonomer, such as propylene, is used as CTA for providing doublebonds, then said copolymerisable comonomer is not calculated to thecomonomer content.

If LDPE polymer is unsaturated, then it has preferably a total amount ofcarbon-carbon double bonds, which originate from vinyl groups,vinylidene groups and trans-vinylene groups, if present, of more than0.4/1000 carbon atoms, preferably of more than 0.5/1000 carbon atoms.The upper limit of the amount of carbon-carbon double bonds present inthe polyolefin is not limited and may preferably be less than 5.0/1000carbon atoms, preferably less than 3.0/1000 carbon atoms.

In some embodiments, e.g. wherein higher crosslinking level with the lowperoxide content is desired, the total amount of carbon-carbon doublebonds, which originate from vinyl groups, vinylidene groups andtrans-vinylene groups, if present, in the unsaturated LDPE, ispreferably higher than 0.40/1000 carbon atoms, preferably higher than0.50/1000 carbon atoms, preferably higher than 0.60/1000 carbon atoms.

If the LDPE is unsaturated LDPE as defined above, it contains preferablyat least vinyl groups and the total amount of vinyl groups is preferablyhigher than 0.05/1000 carbon atoms, still more preferably higher than0.08/1000 carbon atoms, and most preferably of higher than 0.11/1000carbon atoms. Preferably, the total amount of vinyl groups is of lowerthan 4.0/1000 carbon atoms. More preferably, the second polyolefin (b),prior to crosslinking, contains vinyl groups in total amount of morethan 0.20/1000 carbon atoms, still more preferably of more than0.30/1000 carbon atoms.

It is however, preferred if the LDPE of the invention is not unsaturatedand possesses less than 0.2 C═C/1000 C atoms, preferably less than 0.1C═C/1000 C atoms. It is also preferred if the LDPE is a homopolymer. Asthe polymer composition of the invention is not designed forcrosslinking, the presence of unsaturation within the LDPE is notrequired or desired.

The LDPE polymer may have a high melting point, which may be ofimportance especially for a thermoplastic insulation material. Meltingpoints of 112° C. or more are envisaged, such as 114° C. or more,especially 116° C. or more, such as 112 to 125° C.

The LDPE polymer is produced at high pressure by free radical initiatedpolymerisation (referred to as high pressure (HP) radicalpolymerization). The HP reactor can be e.g. a well-known tubular orautoclave reactor or a mixture thereof, preferably a tubular reactor.The high pressure (HP) polymerisation and the adjustment of processconditions for further tailoring the other properties of the polyolefindepending on the desired end application are well known and described inthe literature, and can readily be used by a skilled person. Suitablepolymerisation temperatures range up to 400° C., preferably from 80 to350° C. and pressure from 70 MPa, preferably 100 to 400 MPa, morepreferably from 100 to 350 MPa. Pressure can be measured at least aftercompression stage and/or after the tubular reactor. Temperature can bemeasured at several points during all steps.

After the separation the obtained LDPE is typically in a form of apolymer melt which is normally mixed and pelletized in a pelletisingsection, such as pelletising extruder, arranged in connection to the HPreactor system. Optionally, additive(s), such as antioxidant(s), can beadded in this mixer in a known manner.

Further details of the production of ethylene (co)polymers by highpressure radical polymerization can be found i.a. in the Encyclopedia ofPolymer Science and Engineering, Vol. 6 (1986), pp 383-410 andEncyclopedia of Materials: Science and Technology, 2001 Elsevier ScienceLtd.: “Polyethylene: High-pressure, R. Klimesch, D. Littmann and F.-O.Mähling pp. 7181-7184.

When an unsaturated LDPE copolymer of ethylene is prepared, then, aswell known, the carbon-carbon double bond content can be adjusted bypolymerising the ethylene e.g. in the presence of one or morepolyunsaturated comonomer(s), chain transfer agent(s), or both, usingthe desired feed ratio between monomer, preferably ethylene, andpolyunsaturated comonomer and/or chain transfer agent, depending on thenature and amount of C—C double bonds desired for the unsaturated LDPEcopolymer. I.a. WO 9308222 describes a high pressure radicalpolymerisation of ethylene with polyunsaturated monomers. As a resultthe unsaturation can be uniformly distributed along the polymer chain inrandom copolymerisation manner.

The polymer composition of the invention preferably comprises 95.5 to99.8 wt % of the LDPE. Preferably, the composition comprises 96.0 to99.5 wt %, such as 96.5 to 99.5 wt % of the LDPE, especially 97.0 to99.0 wt %.

High Density Polyethylene Component

The composition of the invention may include a high density polyethylenecomponent which may be unimodal or multimodal. Where the insulationlayer is thermoplastic, a multimodal HDPE is preferably used. Where theinsulation layer is cross-linked, either a unimodal or multimodalinsulation layer can be used, preferably a unimodal HDPE. The polymer isone having a density of at least 940 kg/m³.

The term “multimodal” means herein, unless otherwise stated,multimodality with respect to molecular weight distribution and includestherefore a bimodal polymer. Usually, a polyethylene composition,comprising at least two polyethylene fractions, which have been producedunder different polymerization conditions resulting in different (weightaverage) molecular weights and molecular weight distributions for thefractions, is referred to as “multimodal”. The prefix “multi” relates tothe number of different polymer fractions present in the polymer. Thus,for example, multimodal polymer includes so called “bimodal” polymerconsisting of two fractions. The form of the molecular weightdistribution curve, i.e. the appearance of the graph of the polymerweight fraction as a function of its molecular weight, of a multimodalpolymer will show two or more maxima or is typically distinctlybroadened in comparison with the curves for the individual fractions.For example, if a polymer is produced in a sequential multistageprocess, utilizing reactors coupled in series and using differentconditions in each reactor, the polymer fractions produced in thedifferent reactors will each have their own molecular weightdistribution and weight average molecular weight. When the molecularweight distribution curve of such a polymer is recorded, the individualcurves from these fractions form typically together a broadenedmolecular weight distribution curve for the total resulting polymerproduct.

A unimodal polymer, unless otherwise stated, is unimodal with respect tomolecular weight distribution and therefore contains a single peak on isGPC curve.

The HDPE component (II) of the blend of the invention is preferablypresent in an amount of 0.2 to 4.5 wt %, such as 0.5 to 4.0 wt %,preferably 0.5 to 3.5 wt %, such as 1.0 to 3.0 wt %.

The HDPE preferably has a density according to ISO 1183 at 23° C. of atleast 940 kg/m³, preferably at least 945 kg/m³. The upper limit fordensity may by 980 kg/m³, preferably 975 kg/m³, especially 970 kg/m³. Ahighly preferred density range is 945 to 965 kg/m³, such as 954 to 965kg/m³.

The MFR₂ according to ISO 1133 of the HDPE is preferably in the range of0.1 to 40 g/10 min, preferably 0.25 to 20 g/10 min. Preferably the HDPEhas an MFR₂ of 0.3 to 15 g/10 min. An especially preferred range is 0.4to 15 g/10 min.

In another embodiment, the HDPE may have an MFR₂₁ according to ISO 1133of the HDPE is preferably in the range of 8 to 30 g/10 min, preferably10 to 20 g/10 min.

In some embodiments of the invention, it is preferable if the HDPE is amultimodal polyethylene comprising at least (i) a lower weight averagemolecular weight (LMW) ethylene homopolymer or copolymer component, and(ii) a higher weight average molecular weight (HMW) ethylene homopolymeror copolymer component. Preferably, at least one of said LMW and HMWcomponents is a copolymer of ethylene with at least one comonomer. It ispreferred that at least said HMW component is an ethylene copolymer.Alternatively, if one of said components is a homopolymer, then said LMWis the preferably the homopolymer.

Said LMW component of multimodal polymer preferably has a MFR₂ of atleast 5 g/10 min, preferably at least 50 g/10 min, more preferably atleast 100 g/10 min.

The density of LMW component of said multimodal polymer may range from950 to 980 kg/m³, e.g. 950 to 970 kg/m³.

The LMW component of said multimodal polymer may form from 30 to 70 wt%, e.g. 40 to 60% by weight of the multimodal polymer with the HMWcomponent forming 70 to 30 wt %, e.g. 60 to 40% by weight. In oneembodiment said LMW component forms 50 wt % or more of the multimodalpolymer as defined above or below. Typically, the LMW component forms 45to 55% and the HMW component forms 55 to 45% of the multimodal polymer.

The HMW component of said HDPE has a lower MFR₂ than the LMW component.

The HDPE may be an ethylene homopolymer or copolymer. By ethylenehomopolymer is meant a polymer which is formed essentially only ethylenemonomer units, i.e. is 99.9 wt % ethylene or more. It will beappreciated that minor traces of other monomers may be present due toindustrial ethylene containing trace amounts of other monomers.

The HDPE may also be a copolymer (and is preferably a copolymer) and cantherefore be formed from ethylene with at least one other comonomer,e.g. C₃₋₂₀ olefin. Preferred comonomers are alpha-olefins, especiallywith 3-8 carbon atoms. Preferably, the comonomer is selected from thegroup consisting of propene, 1-butene, 1-hexene, 4-methyl-1-pentene,1-octene, 1,7-octadiene and 7-methyl-1,6-octadiene. The use of 1-hexeneor 1-butene is most preferred.

The HDPE can comprise one monomer or two monomers or more than 2monomers. The use of a single comonomer is preferred. If two comonomersare used it is preferred if one is an C₃₋₈ alpha-olefin and the other isa diene as hereinbefore defined.

The amount of comonomer is preferably such that it comprises 0-3 mol %,more preferably 0.1-2.0 mol % and most preferably 0.1-1.5 mol % of theHDPE. Values under 1.0 mol % are also envisaged, e.g. 0.1 to 1.0 mol %.These can be determined by NMR.

It is preferred however if the ethylene polymer of the inventioncomprises a LMW homopolymer component and a HMW ethylene copolymercomponent, e.g. an ethylene hexene copolymer or an ethylene butenecopolymer.

For the preparation of the HDPE polymerisation methods well known to theskilled person may be used. It is within the scope of the invention fora multimodal, e.g. at least bimodal, polymers to be produced by blendingeach of the components in-situ during the polymerisation process thereof(so called in-situ process) or, alternatively, by blending mechanicallytwo or more separately produced components in a manner known in the art.

Polyethylenes useful in the present invention are preferably obtained byin-situ blending in a multistage polymerisation process. Accordingly,polymers are obtained by in-situ blending in a multistage, i.e. two ormore stage, polymerization process including solution, slurry and gasphase process, in any order. Whilst it is possible to use differentsingle site catalysts in each stage of the process, it is preferred ifthe catalyst employed is the same in both stages.

In a highly preferred embodiment, the HDPE polymer of the invention isprepared by single-site catalysed polymerisation. The use of asingle-site catalysed ethylene polymer gives better conductivity, thanfor example a Ziegler Natta based catalyst. Further, the use of singlesite catalysed polymer allows a lower amount of crosslinking agent to beused to reach the desired degree of crosslinking than for example aZiegler Natta based catalyst. More importantly, the SSC HDPE is a“cleaner” polymer meaning there is less impurity in the cable and lesschance of defects. The HDPE as defined herein may be made using anyconventional single site catalysts, including metallocenes andnon-metallocenes as well known in the field.

Preferably said catalyst is one comprising a metal coordinated by one ormore η-bonding ligands. Such η-bonded metals are typically transitionmetals of Group 3 to 10, e.g. Zr, Hf or Ti, especially Zr or Hf. Theη-bonding ligand is typically an η-cyclic ligand, i.e. a homo orheterocyclic cyclopentadienyl group optionally with fused or pendantsubstituents. Such single site, preferably metallocene, procatalystshave been widely described in the scientific and patent literature forabout twenty years. Procatalyst refers herein to said transition metalcomplex.

The metallocene procatalyst may have a formula II:

(Cp)_(m) R _(n) MX _(q)  (II)

wherein:

each Cp independently is an unsubstituted or substituted and/or fusedhomo- or heterocyclopentadienyl ligand, e.g. substituted orunsubstituted cyclopentadienyl, substituted or unsubstituted indenyl orsubstituted or unsubstituted fluorenyl ligand;

the optional one or more substituent(s) being independently selectedpreferably from halogen, hydrocarbyl (e.g. C₁-C₂₀-alkyl, C₂-C₂₀-alkenyl,C₂-C₂₀-alkynyl, C₃-C₁₂-cycloalkyl, C6-C₂₀-aryl or C₇-C₂₀-arylalkyl),C₃-C₁₂-cycloalkyl which contains 1, 2, 3 or 4 heteroatom(s) in the ringmoiety, C₆-C₂₀-heteroaryl, C₁-C₂₀-haloalkyl, —SiR″₃, —OSiR″₃, —SR″,—PR″₂ or —NR″₂,

each R″ is independently a hydrogen or hydrocarbyl, e.g. C₁-C₂₀-alkyl,C₂-C₂₀-alkenyl, C₂-C₂₀-alkynyl, C₃-C₁₂-cycloalkyl or C₆-C₂₀-aryl; ore.g. in case of —NR″₂, the two substituents R″ can form a ring, e.g.five- or six-membered ring, together with the nitrogen atom to whichthey are attached;

R is a bridge of 1-7 atoms, e.g. a bridge of 1-4 C-atoms and 0-4heteroatoms, wherein the heteroatom(s) can be e.g. Si, Ge and/or Oatom(s), wherein each of the bridge atoms may bear independentlysubstituents, such as C₁₋₂₀-alkyl, tri(C₁₋₂₀-alkyl)silyl,tri(C₁₋₂₀-alkyl)siloxy or C₆₋₂₀-aryl substituents); or a bridge of 1-3,e.g. one or two, hetero atoms, such as silicon, germanium and/or oxygenatom(s), e.g. —SiR¹ ₂—, wherein each R¹ is independently C₁₋₂₀-alkyl,C₆₋₂₀-aryl or tri(C₁₋₂₀-alkyl)silyl- residue, such as trimethylsilyl;

M is a transition metal of Group 3 to 10, preferably of Group 4 to 6,such as Group 4, e.g. Ti, Zr or Hf, especially Hf;

each X is independently a sigma-ligand, such as H, halogen, C₁₋₂₀-alkyl,C₁₋₂₀-alkoxy, C₂-C₂₀-alkenyl, C₂-C₂₀-alkynyl, C3-C12-cycloalkyl,C6-C₂₀-aryl, C6-C₂₀-aryloxy, C7-C₂₀-arylalkyl, C7-C20-arylalkenyl, —SR″,—PR″₃, —SiR″₃, —OSiR″₃, —NR″₂ or —CH₂—Y, wherein Y is C6-C20-aryl,C6-C20-heteroaryl, C1-C₂₀-alkoxy, C6-C20-aryloxy, NR″₂, —SR″, —PR″₃,—SiR″₃, or —OSiR″₃;

each of the above mentioned ring moieties alone or as a part of anothermoiety as the substituent for Cp, X, R″ or R1 can further be substitutede.g. with C1-C20-alkyl which may contain Si and/or O atoms;

n is 0, 1 or 2, e.g. 0 or 1,

m is 1, 2 or 3, e.g. 1 or 2,

q is 1, 2 or 3, e.g. 2 or 3,

wherein m+q is equal to the valency of M.

Suitably, in each X as —CH₂—Y, each Y is independently selected fromC6-C20-aryl, NR″₂, —SiR″₃ or —OSiR″₃. Most preferably, X as —CH₂—Y isbenzyl. Each X other than —CH₂—Y is independently halogen, C1-C20-alkyl,C1-C20-alkoxy, C6-C20-aryl, C7-C20-arylalkenyl or —NR″₂ as definedabove, e.g. —N(C1-C20-alkyl)₂.

Preferably, q is 2, each X is halogen or —CH₂—Y, and each Y isindependently as defined above.

Cp is preferably cyclopentadienyl, indenyl, tetrahydroindenyl orfluorenyl, optionally substituted as defined above.

In a suitable subgroup of the compounds of formula II, each Cpindependently bears 1, 2, 3 or 4 substituents as defined above,preferably 1, 2 or 3, such as 1 or 2 substituents, which are preferablyselected from C1-C20-alkyl, C6-C20-aryl, C7-C20-arylalkyl (wherein thearyl ring alone or as a part of a further moiety may further besubstituted as indicated above), —OSiR″₃, wherein R″ is as indicatedabove, preferably C1-C20-alkyl.

R, if present, is preferably a methylene, ethylene or a silyl bridge,whereby the silyl can be substituted as defined above, e.g. a(dimethyl)Si═, (methylphenyl)Si═ or (trimethylsilylmethyl)Si═; n is 0 or1; m is 2 and q is two. Preferably, R″ is other than hydrogen.

A specific subgroup includes the well known metallocenes of Zr, Hf andTi with two η-5-ligands which may be bridged or unbridgedcyclopentadienyl ligands optionally substituted with e.g. siloxy, oralkyl (e.g. C1-6-alkyl) as defined above, or with two unbridged orbridged indenyl ligands optionally substituted in any of the ringmoieties with e.g. siloxy or alkyl as defined above, e.g. at 2-, 3-, 4-and/or 7-positions. Preferred bridges are ethylene or —SiMe₂.

The preparation of the metallocenes can be carried out according oranalogously to the methods known from the literature and is withinskills of a person skilled in the field. Thus for the preparation seee.g. EP-A-129 368, examples of compounds wherein the metal atom bears a—NR″₂ ligand see i.a. in WO-A-9856831 and WO-A-0034341. For thepreparation see also e.g. in EP-A-260 130, WO-A-9728170, WO-A-9846616,WO-A-9849208, WO-A-9912981, WO-A-9919335, WO-A-9856831, WO-A-00/34341,EP-A-423 101 and EP-A-537 130.

Alternatively, in a further subgroup of the metallocene compounds, themetal bears a Cp group as defined above and additionally a ƒ1 or η2ligand, wherein said ligands may or may not be bridged to each other.Such compounds are described e.g. in WO-A-9613529, the contents of whichare incorporated herein by reference.

Further preferred metallocenes include those of formula (I)

Cp′ ₂ M′X′ ₂

wherein each X′ is halogen, C₁₋₆ alkyl, benzyl or hydrogen;

M′ is Hf or Zr;

Cp′ is a cyclopentadienyl or indenyl group optionally substituted by aC₁₋₁₀ hydrocarbyl group or groups and being optionally bridged, e.g. viaan ethylene or dimethylsilyl link.

Especially preferred catalysts are bis-(n-butyl cyclopentadienyl)hafnium dibenzyl, and bis-(n-butyl cyclopentadienyl) zirconiumdichloride.

Metallocene procatalysts are generally used as part of a catalyst systemwhich also includes a catalyst activator, called also as cocatalyst.Useful activators are, among others, aluminium compounds, like aluminiumalkoxy compounds. Suitable aluminium alkoxy activators are for examplemethylaluminoxane (MAO), hexaisobutylaluminoxane andtetraisobutylaluminoxane. In addition boron compounds (e.g. afluoroboron compound such as triphenylpentafluoroboron ortriphentylcarbenium tetraphenylpentafluoroborate ((C₆H₅)₃B+B—(C₆F₅)₄))can be used as activators. The cocatalysts and activators and thepreparation of such catalyst systems is well known in the field. Forinstance, when an aluminium alkoxy compound is used as an activator, theAl/M molar ratio of the catalyst system (Al is the aluminium from theactivator and M is the transition metal from the transition metalcomplex) is suitable from 50 to 500 mol/mol, preferably from 100 to 400mol/mol. Ratios below or above said ranges are also possible, but theabove ranges are often the most useful.

If desired the procatalyst, procatalyst/cocatalyst mixture or aprocatalyst/cocatalyst reaction product may be used in supported form(e.g. on a silica or alumina carrier), unsupported form or it may beprecipitated and used as such. One feasible way for producing thecatalyst system is based on the emulsion technology, wherein no externalsupport is used, but the solid catalyst is formed from by solidificationof catalyst droplets dispersed in a continuous phase. The solidificationmethod and further feasible metallocenes are described e.g. inWO03/051934 which is incorporated herein as a reference.

It is also possible to use combinations of different activators andprocatalysts. In addition additives and modifiers and the like can beused, as is known in the art.

Any catalytically active catalyst system including the procatalyst, e.g.metallocene complex, is referred herein as single site or metallocenecatalyst (system).

Ideally therefore, the HDPE used in the blend of the invention areproduced in at least two-stage polymerization using a single sitecatalyst catalyst. Thus, for example two slurry reactors or two gasphase reactors, or any combinations thereof, in any order can beemployed. Preferably however, the polyethylene is made using a slurrypolymerization in a loop reactor followed by a gas phase polymerizationin a gas phase reactor.

A loop reactor-gas phase reactor system is well known as Borealistechnology, i.e. as a BORSTAR™ reactor system. Such a multistage processis disclosed e.g. in EP517868.

The conditions used in such a process are well known. For slurryreactors, the reaction temperature will generally be in the range 60 to110° C., e.g. 85-110° C., the reactor pressure will generally be in therange 5 to 80 bar, e.g. 50-65 bar, and the residence time will generallybe in the range 0.3 to 5 hours, e.g. 0.5 to 2 hours. The diluent usedwill generally be an aliphatic hydrocarbon having a boiling point in therange −70 to +100° C., e.g. propane. In such reactors, polymerizationmay if desired be effected under supercritical conditions. Slurrypolymerisation may also be carried out in bulk where the reaction mediumis formed from the monomer being polymerised.

For gas phase reactors, the reaction temperature used will generally bein the range 60 to 115° C., e.g. 70 to 110° C., the reactor pressurewill generally be in the range 10 to 25 bar, and the residence time willgenerally be 1 to 8 hours. The gas used will commonly be a non-reactivegas such as nitrogen or low boiling point hydrocarbons such as propanetogether with monomer, e.g. ethylene.

The ethylene concentration in the first, preferably loop, reactor may bearound 5 to 15 mol %, e.g. 7.5 to 12 mol %.

In the second, preferably gas phase, reactor, ethylene concentration ispreferably much higher, e.g. at least 40 mol % such as 45 to 65 mol %,preferably 50 to 60 mol %.

Preferably, the first polymer fraction is produced in a continuouslyoperating loop reactor where ethylene is polymerised in the presence ofa polymerization catalyst as stated above and a chain transfer agentsuch as hydrogen. The diluent is typically an inert aliphatichydrocarbon, preferably isobutane or propane. The reaction product isthen transferred, preferably to continuously operating gas phasereactor. The second component can then be formed in a gas phase reactorusing preferably the same catalyst.

The HDPE is a commercial product and can be purchased from varioussuppliers.

Polymer Composition

The polymer composition of use in the insulation layer of the inventioncomprises the HDPE and LDPE mixed in ratios as herein before defined.The polymer composition preferably consists essentially of thecomponents (I) and (II). The term consists essentially of implies thatthere are no other polymer components present in the composition. Itwill be appreciated that the polymer composition may contain standardpolymer additives discussed in more detail below. The term consistsessentially of is used to exclude the presence of other polymercomponents but is intended to allow the option of additives beingpresent.

During manufacture of the composition, the components can be blended andhomogenously mixed, e.g. melt mixed in an extruder.

Conductivity

In all embodiments of the invention it is preferred if the conductivityof the polymer composition or the conductivity of the insulation layeris 1.5 fS/m or less when determined using method A.

As we demonstrate in the examples, in a crosslinked insulation layer,where the LDPE is combined with a low amount of HDPE, such as 1 to 3 wt%, the resulting blend has a remarkably low conductivity, e.g. 1.0 fS/mmeasured via DC method A. In contrast, when 5 wt % is used a much highervalue is measured (see WO2011/113686). Low HDPE content is thereforesurprisingly associated with lower conductivity in crosslinkedmaterials. Lower HDPE also means less impurity and hence less change ofdefects in the insulation layer.

In thermoplastic applications, lower HDPE offers comparable conductivityto higher HDPE percentages and better purity. Where the HDPE ismultimodal, lower conductivity is observed in thermoplastic embodiments.

In all embodiments, it is preferred if the conductivity of the polymercomposition of the insulation layer is 3.5×10⁻¹⁷ S/cm or less whenmeasured according to DC conductivity method as described under“Determination Methods B”, such as 3.0×10⁻¹⁷ S/cm or less.

We have also found that in the context of a thermoplastic blend, the useof a bimodal material as the HDPE leads to lower conductivity.

Cables

The cable of the invention is preferably a DC cable. A DC power cable isdefined to be a DC cable transferring energy operating at any voltagelevel, typically operating at voltages higher than 1 kV. The DC powercable can be a low voltage (LV), a medium voltage (MV), a high voltage(HV) or an extra high voltage (EHV) DC cable, which terms, as wellknown, indicate the level of operating voltage. The polymer is even morepreferable used in the insulation layer for a DC power cable operatingat voltages higher than 36 kV, such as a HV DC cable. For HV DC cablesthe operating voltage is defined herein as the electric voltage betweenground and the conductor of the high voltage cable.

Preferably the HV DC power cable of the invention is one operating atvoltages of 40 kV or higher, even at voltages of 50 kV or higher. Morepreferably, the HV DC power cable operates at voltages of 60 kV orhigher. The invention is also highly feasible in very demanding cableapplications and further cables of the invention are HV DC power cableoperating at voltages higher than 70 kV. Voltages of 100 kV or more aretargeted, such as 200 kV or more, more preferably 300 KV or more,especially 400 kV or more, more especially 500 kV or more. Voltages of640 KV or more, such as 700 kV are also envisaged. The upper limit isnot limited. The practical upper limit can be up to 1500 kV such as upto 1100 kV. The cables of the invention operate well therefore indemanding extra HV DC power cable applications operating 400 to 850 kV,such as 650 to 850 kV.

A cable, such as a DC cable, comprises an inner semiconductive layercomprising a first semiconductive composition, an insulation layercomprising the polymer composition of the invention and an outersemiconductive layer comprising a second semiconductive composition, inthat order.

The polymer composition of the invention is used in the insulation layerof the cable. Ideally, the insulation layer comprises at least 95 wt %,such as at least 98 wt % of the polymer composition of the invention,such as at least 99 wt %. It is preferred therefore if the polymercomposition of the invention is the only non-additive component used inthe insulation layer of the cables of the invention. Thus, it ispreferred if the insulation layer consists essentially of thecomposition of the invention. The term consists essentially of is usedherein to mean that the only polymer composition present is thatdefined. It will be appreciated that the insulation layer may containstandard polymer additives such as scorch retarders, water treeretarders, antioxidants and so on. These are not excluded by the term“consists essentially of”. Note also that these additives may be addedas part of a masterbatch and hence carried on a polymer carrier. The useof masterbatch additives is not excluded by the term consistsessentially of.

The insulation layer can have a beneficial low electrical conductivitywhen it is cross-linked with a crosslinking agent. The insulation layerof the cables of the invention can thus optionally be crosslinkable.

The term crosslinkable means that the insulation layer can becross-linked using a crosslinking agent before use. The insulation layerwill need to comprise a crosslinking agent in order to be crosslinkable,typically a free radical generating agent. The cross-linked polymercomposition has a typical network, i.a. interpolymer crosslinks(bridges), as well known in the field.

If the insulation layer is cross-linked, any parameter of the insulationlayer other than conductivity is ideally measured before thecrosslinking unless otherwise indicated. In embodiments, wherein theinsulation layer comprises no crosslinking agent, the electricalconductivity as described under the “Determination method” is measuredfrom a sample of polymer forming the insulation layer which isnon-cross-linked (i.e. does not contain a crosslinking agent and has notbeen cross-linked with a crosslinking agent). In embodiments, whereinthe insulation layer is cross-linked with a crosslinking agent, then theelectrical conductivity is measured from a sample of the cross-linkedpolymer (i.e. a sample of the polymer is first cross-linked with thecrosslinking agent initially present and then the electricalconductivity is measured from the obtained cross-linked sample). Theconductivity measurement from a non-cross-linked or a cross-linkedpolymer composition sample is described under “Determination Methods”.

The amount of the crosslinking agent used, if present, can vary,preferably within the ranges given below. Preferably a peroxide is usedin an amount of 0 to 110 mmol —O—O—/kg polymer composition of theinsulation layer, preferably 0 to 90 mmol —O—O—/kg polymer composition(corresponds 0 to 2.4 wt % of dicumyl peroxide based on the polymercomposition), preferably of 0 to 37 mmol —O—O—/kg polymer composition,preferably of 0 to 35 mmol —O—O—/kg polymer composition, preferably of 0to 34 mmol —O—O—/kg polymer composition, preferably of 0 to 33 mmol—O—O—/kg polymer composition, more preferably from 0 to 30 mmol —O—O—/kgpolymer composition, more preferably from 0 to 20 mmol —O—O—/kg polymercomposition, more preferably from 0 to 10.0 mmol —O—O—/kg polymercomposition, more preferably from 0 to 7.0 mmol —O—O—/kg polymercomposition, more preferably less than 5.0 mmol —O—O—/kg polymercomposition, most preferably the polymer composition comprises nocrosslinking agent (=0 wt % of added crosslinking agent). The insulationlayer is thus ideally free of byproducts of the decomposition of theperoxide.

The lower limit of the crosslinking agent, if present, is not limitedand can be at least 0.1 mmol —O—O—/kg polymer composition in theinsulation layer, preferably at least 0.5 mmol —O—O—/kg polymercomposition, more preferably at least 5.0 mmol —O—O—/kg polymercomposition. The lower peroxide content can shorten the requireddegassing step of the produced and cross-linked cable, if desired.

The unit “mmol —O—O—/kg polymer composition” means herein the content(mmol) of peroxide functional groups per kg polymer composition, whenmeasured from the polymer composition prior to crosslinking. Forinstance the 35 mmol —O—O—/kg polymer composition corresponds to 0.95 wt% of the well-known dicumyl peroxide based on the total amount (100 wt%) of the polymer composition.

Such polymer composition may comprise one type of peroxide or two ormore different types of peroxide, in which case the amount (in mmol) of—O—O—/kg polymer composition, as defined above, below or in claims, isthe sum of the amount of —O—O—/kg polymer composition of each peroxidetype. As non-limiting examples of suitable organic peroxides,di-tert-amylperoxide, 2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne,2,5-di(tert-butylperoxy)-2,5-dimethylhexane, tert-butylcumylperoxide,di(tert-butyl)peroxide, dicumylperoxide,butyl-4,4-bis(tert-butylperoxy)-valerate,1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane,tert-butylperoxybenzoate, dibenzoylperoxide, bis(tertbutylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane,1,1-di(tert-butylperoxy)cyclohexane, 1,1-di(tert amylperoxy)cyclohexane,or any mixtures thereof, can be mentioned. Preferably, the peroxide isselected from 2,5-di(tert-butylperoxy)-2,5-dimethylhexane,di(tert-butylperoxyisopropyl)benzene, dicumylperoxide,tert-butylcumylperoxide, di(tert-butyl)peroxide, or mixtures thereof.Most preferably, the peroxide is dicumylperoxide.

Where the insulation layer is not crosslinked, it is preferred that theinsulation layer comprises no crosslinking agent. The prior artdrawbacks relating to the use of a crosslinking agent in cable layer cantherefore be avoided. Naturally, the non cross-linked embodiment alsosimplifies the cable production process. As no crosslinking agent isrequired, the raw material costs are lower. Also, it is generallyrequired to degas a cross-linked cable layer to remove the by-productsof the peroxide after crosslinking Where the material is notcross-linked, no such degassing step is required.

The insulation layer may contain, in addition to the LDPE blend and theoptional peroxide, further component(s) such as additives (such as anyof antioxidant(s), scorch retarder(s) (SR), crosslinking booster(s),stabiliser(s), processing aid(s), flame retardant additive(s), watertree retardant additive(s), acid or ion scavenger(s), inorganicfiller(s) and voltage stabilizer(s), as known in the polymer field.

The insulation layer may therefore comprise conventionally usedadditive(s) for W&C applications, such as one or more antioxidant(s) andoptionally one or more scorch retarder(s), preferably at least one ormore antioxidant(s). The used amounts of additives are conventional andwell known to a skilled person, e.g. 0.1 to 1.0 wt %.

As non-limiting examples of antioxidants e.g. sterically hindered orsemi-hindered phenols, aromatic amines, aliphatic sterically hinderedamines, organic phosphites or phosphonites, thio compounds, and mixturesthereof, can be mentioned.

Preferably, the insulation layer does not comprise a carbon black. Alsopreferably, the insulation layer does not comprise flame retardingadditive(s), e.g. a metal hydroxide containing additives in flameretarding amounts.

The cable of the invention also contains inner and outer semiconductivelayers. These can be made of any conventional material suitable for usein these layers. The inner and the outer semiconductive compositions canbe different or identical and may comprise a polymer(s) which ispreferably a polyolefin or a mixture of polyolefins and a conductivefiller, preferably carbon black. Suitable polyolefin(s) are e.g.polyethylene produced in a low pressure process or a polyethyleneproduced in a HP process (LDPE). The carbon black can be anyconventional carbon black used in the semiconductive layers of a DCpower cable, preferably in the semiconductive layer of a DC power cable.Preferably the carbon black has one or more of the following properties:a) a primary particle size of at least 5 nm which is defined as thenumber average particle diameter according ASTM D3849-95a, dispersionprocedure D b) iodine number of at least 30 mg/g according to ASTMD1510, c) oil absorption number of at least 30 ml/100 g which ismeasured according to ASTM D2414. Non-limiting examples of carbon blacksare e.g. acetylene carbon black, furnace carbon black and Ketjen carbonblack, preferably furnace carbon black and acetylene carbon black.Preferably, the polymer composition comprises 10 to 50 wt % carbonblack, based on the weight of the Semiconductive composition.

In a preferable embodiment, the outer semiconductive layer iscross-linked. In another preferred embodiment, the inner semiconductivelayer is preferably non-cross-linked. Overall therefore it is preferredif the inner semiconductive layer and the insulation layer remain noncross-linked where the outer semiconductive layer is cross-linked. Aperoxide crosslinking agent can therefore be provided in the outersemiconductive layer only.

The cable comprises one or more conductors, e.g. wires. Preferably theconductor is an electrical conductor and comprises one or more metalwires. Cu wire is preferred.

As well known the cable can optionally comprise further layers, e.g.screen(s), a jacketing layer(s), other protective layer(s) or anycombinations thereof.

Cable Manufacture

The invention also provides a process for producing a cable comprisingthe steps of

-   -   applying on a conductor, preferably by (co)extrusion, an inner        semiconductive layer, an insulation layer and an outer        semiconductive layer, in that order, wherein the insulation        layer comprises the composition of the invention.

The process may optionally comprise the steps of crosslinking one orboth of the inner semiconductive layer or outer semiconductive layer,without crosslinking the insulation layer. Preferably however, no layeris cross-linked. The cable of the invention is ideally thermoplastic.More preferably, a cable is produced, wherein the process comprises thesteps of

(a)—providing and mixing, preferably melt mixing in an extruder, anoptionally crosslinkable first semiconductive composition comprising apolymer, a carbon black and optionally further component(s) for theinner semiconductive layer,

providing and mixing, preferably melt mixing in an extruder, the polymercomposition of the invention; and

providing and mixing, preferably melt mixing in an extruder, a secondsemiconductive composition which is optionally crosslinkable andcomprises a polymer, a carbon black and optionally further component(s)for the outer semiconductive layer,

(b) applying on one or more conductors, preferably by coextrusion,

a melt mix of the first semiconductive composition obtained from step(a) to form the inner semiconductive layer,

a meltmix of polymer composition of the invention obtained from step (a)to form the insulation layer, and

a meltmix of the second semiconductive composition obtained from step(a) to form the outer semiconductive layer, and

(c) optionally crosslinking at crosslinking conditions one or both ofthe first semiconductive composition of the inner semiconductive layerand the second semiconductive composition of the outer semiconductivelayer, of the obtained cable, without crosslinking the insulation layer.

Preferably in step (c) the second semiconductive polymer composition ofthe outer semiconductive layer is cross-linked. Also preferably, thesecond semiconductive polymer composition of the outer semiconductivelayer is cross-linked, without crosslinking the insulation layer or thefirst semiconductive composition of the inner semiconductive layer.

Melt mixing means mixing above the melting point of at least the majorpolymer component(s) of the obtained mixture and is carried out forexample, without limiting to, in a temperature of at least 15° C. abovethe melting or softening point of polymer component(s).

The term “(co)extrusion” means herein that in case of two or morelayers, said layers can be extruded in separate steps, or at least twoor all of said layers can be coextruded in a same extrusion step, aswell known in the art. The term “(co)extrusion” means herein also thatall or part of the layer(s) are formed simultaneously using one or moreextrusion heads. For instance a triple extrusion can be used for formingthree layers. In case a layer is formed using more than one extrusionheads, then for instance, the layers can be extruded using two extrusionheads, the first one for forming the inner semiconductive layer and theinner part of the insulation layer, and the second head for forming theouter insulation layer and the outer semiconductive layer.

As well known, the polymer composition of the invention and the optionaland preferred first and second semiconductive compositions can beproduced before or during the cable production process.

Preferably, the polymers required to manufacture the cable of theinvention are provided to the cable production process in form ofpowder, grain or pellets. Pellets mean herein generally any polymerproduct which is formed from reactor-made polymer (obtained directlyfrom the reactor) by post-reactor modification to solid polymerparticles.

Accordingly, the LDPE and the component (II) material can be premixed,e.g. melt mixed together and pelletized, before mixing. Alternatively,and preferably, these components can be provided in separate pellets tothe (melt) mixing step (a), where the pellets are blended together.

The (melt) mixing step (a) of the provided polymer composition of theinvention and of the preferable first and second semiconductivecompositions is preferably carried out in a cable extruder. The step a)of the cable production process may optionally comprise a separatemixing step, e.g. in a mixer arranged in connection and preceding thecable extruder of the cable production line. Mixing in the precedingseparate mixer can be carried out by mixing with or without externalheating (heating with an external source) of the component(s).

Any crosslinking agent can be added before the cable production processor during the (melt) mixing step (a). For instance, and preferably, thecrosslinking agent and also the optional further component(s), such asadditive(s), can already be present in the polymers used. Thecrosslinking agent is added, preferably impregnated, onto the solidpolymer particles, preferably pellets.

It is preferred that the melt mix of the polymer composition obtainedfrom (melt)mixing step (a) consists of the LDPE (I) and second component(II) as the sole polymer component(s). The optional and preferableadditive(s) can be added to polymer composition as such or as a mixturewith a carrier polymer, i.e. in a form of so-called master batch.

The crosslinking of other layers can be carried out at increasedtemperature which is chosen, as well known, depending on the type ofcrosslinking agent. For instance temperatures above 150° C., such asfrom 160 to 350° C., are typical, however without limiting thereto.

The processing temperatures and devices are well known in the art, e.g.conventional mixers and extruders, such as single or twin screwextruders, are suitable for the process of the invention.

The nature of the cooling process is important in governing theproperties of the insulation layer. Once the polymer material has beenextruded the polymer composition needs to cool. This can be effectedquickly or slowly depending on the conditions applied to the cable.

The inventors have found that rapid quenching maximizes the formation ofco-crystals, whereas slow cooling increasingly favors segregation ofLDPE and HDPE and thus the formation of pure crystalline domains.Improved creep properties are generally observed with slower cooling. Itis thus a further object of the invention if the composition of theinsulation layer is cooled at a rate of less than 5° C./min.

The thickness of the insulation layer of the cable, more preferably ofthe DC power cable such as HV DC power cable, is typically 2 mm or more,preferably at least 3 mm, preferably of at least 5 to 100 mm, morepreferably from 5 to 50 mm, and conventionally 5 to 40 mm, e.g. 5 to 35mm, when measured from a cross section of the insulation layer of thecable.

The thickness of the inner and outer semiconductive layers is typicallyless than that of the insulation layer, and in HV DC power cables can bee.g. more than 0.1 mm, such as from 0.3 up to 20 mm, 0.3 to 10 of innersemiconductive and outer semiconductive layer. The thickness of theinner semiconductive layer is preferably 0.3-5.0 mm, preferably 0.5-3.0mm, preferably 0.8-2.0 mm. The thickness of the outer semiconductivelayer is preferably from 0.3 to 10 mm, such as 0.3 to 5 mm, preferably0.5 to 3.0 mm, preferably 0.8-3.0 mm. It is evident for and within theskills of a skilled person that the thickness of the layers of the DCcable depends on the intended voltage level of the end application cableand can be chosen accordingly.

The preferable embodiments of the invention can be combined with eachother in any way to further define the invention.

The invention will now be described with reference to the following nonlimiting examples.

Determination Methods

Unless otherwise stated in the description or experimental part thefollowing methods were used for the property determinations.

Wt %: % by weight

Melt Flow Rate

The melt flow rate (MFR) is determined according to ISO 1133 and isindicated in g/10 min. The MFR is an indication of the flowability, andhence the processability, of the polymer. The higher the melt flow rate,the lower the viscosity of the polymer. The MFR is determined at 190° C.for polyethylene and at 230° C. for polypropylene. MFR may be determinedat different loadings such as 2.16 kg (MFR₂) or 21.6 kg (MFR₂₁).

Molecular Weight

Mz, Mw, Mn, and MWD are measured by Gel Permeation Chromatography (GPC)according to the following method:

The weight average molecular weight Mw and the molecular weightdistribution (MWD=Mw/Mn wherein Mn is the number average molecularweight and Mw is the weight average molecular weight; Mz is thez-average molecular weight) is measured according to ISO 16014-4:2003and ASTM D 6474-99. A Waters GPCV2000 instrument, equipped withrefractive index detector and online viscosimeter was used with2×GMHXL-HT and 1×G7000HXL-HT TSK-gel columns from Tosoh Bioscience and1,2,4-trichlorobenzene (TCB, stabilized with 250 mg/L 2,6-Ditert-butyl-4-methyl-phenol) as solvent at 140° C. and at a constant flowrate of 1 mL/min. 209.5 μL of sample solution were injected peranalysis. The column set was calibrated using universal calibration(according to ISO 16014-2:2003) with at least 15 narrow MWD polystyrene(PS) standards in the range of 1 kg/mol to 12 000 kg/mol. Mark Houwinkconstants were used as given in ASTM D 6474-99. All samples wereprepared by dissolving 0.5-4.0 mg of polymer in 4 mL (at 140° C.) ofstabilized TCB (same as mobile phase) and keeping for max. 3 hours at amaximum temperature of 160° C. with continuous gentle shaking priorsampling in into the GPC instrument.

Comonomer Contents a) Comonomer Content in Random Copolymer ofPolypropylene:

Quantitative Fourier transform infrared (FTIR) spectroscopy was used toquantify the amount of comonomer. Calibration was achieved bycorrelation to comonomer contents determined by quantitative nuclearmagnetic resonance (NMR) spectroscopy.

The calibration procedure based on results obtained from quantitative¹³C-NMR spectroscopy was undertaken in the conventional manner welldocumented in the literature.

The amount of comonomer (N) was determined as weight percent (wt %) via:

N=k1(A/R)+k2

wherein A is the maximum absorbance defined of the comonomer band, R themaximum absorbance defined as peak height of the reference peak and withk1 and k2 the linear constants obtained by calibration. The band usedfor ethylene content quantification is selected depending if theethylene content is random (730 cm⁻¹) or block-like (as in heterophasicPP copolymer) (720 cm⁻¹). The absorbance at 4324 cm⁻¹ was used as areference band.

b) Quantification of Alpha-Olefin Content in Linear Low DensityPolyethylenes and Low Density Polyethylenes by NMR Spectroscopy:

The comonomer content was determined by quantitative 13 C nuclearmagnetic resonance (NMR) spectroscopy after basic assignment (J. RandallJMS—Rev. Macromol. Chem. Phys., C29(2&3), 201-317 (1989). Experimentalparameters were adjusted to ensure measurement of quantitative spectrafor this specific task.

Specifically solution-state NMR spectroscopy was employed using a BrukerAvanceIII 400 spectrometer. Homogeneous samples were prepared bydissolving approximately 0.200 g of polymer in 2.5 ml ofdeuterated-tetrachloroethene in 10 mm sample tubes utilising a heatblock and rotating tube oven at 140 C. Proton decoupled 13 C singlepulse NMR spectra with NOE (powergated) were recorded using thefollowing acquisition parameters: a flip-angle of 90 degrees, 4 dummyscans, 4096 transients an acquisition time of 1.6 s, a spectral width of20 kHz, a temperature of 125 C, a bilevel WALTZ proton decoupling schemeand a relaxation delay of 3.0 s. The resulting FID was processed usingthe following processing parameters: zero-filling to 32k data points andapodisation using a gaussian window function; automatic zeroth and firstorder phase correction and automatic baseline correction using a fifthorder polynomial restricted to the region of interest.

Quantities were calculated using simple corrected ratios of the signalintegrals of representative sites based upon methods well known in theart.

c) Comonomer Content of Polar Comonomers in Low Density Polyethylene (1)Polymers Containing >6 wt % Polar Comonomer Units

Comonomer content (wt %) was determined in a known manner based onFourier transform infrared spectroscopy (FTIR) determination calibratedwith quantitative nuclear magnetic resonance (NMR) spectroscopy. Belowis exemplified the determination of the polar comonomer content ofethylene ethyl acrylate, ethylene butyl acrylate and ethylene methylacrylate. Film samples of the polymers were prepared for the FTIRmeasurement: 0.5-0.7 mm thickness was used for ethylene butyl acrylateand ethylene ethyl acrylate and 0.10 mm film thickness for ethylenemethyl acrylate in amount of >6 wt %. Films were pressed using a Specacfilm press at 150° C., approximately at 5 tons, 1-2 minutes, and thencooled with cold water in a not controlled manner. The accuratethickness of the obtained film samples was measured.

After the analysis with FTIR, base lines in absorbance mode were drawnfor the peaks to be analysed. The absorbance peak for the comonomer wasnormalised with the absorbance peak of polyethylene (e.g. the peakheight for butyl acrylate or ethyl acrylate at 3450 cm⁻¹ was dividedwith the peak height of polyethylene at 2020 cm⁻¹). The NMR spectroscopycalibration procedure was undertaken in the conventional manner which iswell documented in the literature, explained below.

For the determination of the content of methyl acrylate a 0.10 mm thickfilm sample was prepared. After the analysis the maximum absorbance forthe peak for the methylacrylate at 3455 cm⁻¹ was subtracted with theabsorbance value for the base line at 2475 cm⁻¹(A_(methylacrylate)−A₂₄₇₅). Then the maximum absorbance peak for thepolyethylene peak at 2660 cm⁻¹ was subtracted with the absorbance valuefor the base line at 2475 cm⁻¹ (A₂₆₆₀−A₂₄₇₅). The ratio between(A_(methylacrylate)−A₂₄₇₅) and (A₂₆₆₀−A₂₄₇₅) was then calculated in theconventional manner which is well documented in the literature.

The weight-% can be converted to mol-% by calculation. It is welldocumented in the literature.

Quantification of Copolymer Content in Polymers by NMR Spectroscopy

The comonomer content was determined by quantitative nuclear magneticresonance (NMR) spectroscopy after basic assignment (e.g. “NMR Spectraof Polymers and Polymer Additives”, A. J. Brandolini and D. D. Hills,2000, Marcel Dekker, Inc. New York). Experimental parameters wereadjusted to ensure measurement of quantitative spectra for this specifictask (e.g “200 and More NMR Experiments: A Practical Course”, S. Bergerand S. Braun, 2004, Wiley-VCH, Weinheim). Quantities were calculatedusing simple corrected ratios of the signal integrals of representativesites in a manner known in the art.

(2) Polymers Containing 6 wt. % or Less Polar Comonomer Units

Comonomer content (wt. %) was determined in a known manner based onFourier transform infrared spectroscopy (FTIR) determination calibratedwith quantitative nuclear magnetic resonance (NMR) spectroscopy. Belowis exemplified the determination of the polar comonomer content ofethylene butyl acrylate and ethylene methyl acrylate. For the FT-IRmeasurement a film samples of 0.05 to 0.12 mm thickness were prepared asdescribed above under method 1). The accurate thickness of the obtainedfilm samples was measured.

After the analysis with FT-IR base lines in absorbance mode were drawnfor the peaks to be analysed. The maximum absorbance for the peak forthe comonomer (e.g. for methylacrylate at 1164 cm⁻¹ and butylacrylate at1165 cm⁻¹) was subtracted with the absorbance value for the base line at1850 cm⁻¹ (A_(polar comonomer)−A₁₈₅₀). Then the maximum absorbance peakfor polyethylene peak at 2660 cm⁻¹ was subtracted with the absorbancevalue for the base line at 1850 cm⁻¹ (A₂₆₆₀−A₁₈₅₀). The ratio between(A_(comonomer)−A₁₈₅₀) and (A₂₆₆₀−A₁₈₅₀) was then calculated. The NMRspectroscopy calibration procedure was undertaken in the conventionalmanner which is well documented in the literature, as described aboveunder method 1).

The weight-% can be converted to mol-% by calculation. It is welldocumented in the literature.

Below is exemplified how polar comonomer content obtained from the abovemethod (1) or (2), depending on the amount thereof, can be converted tomicromol or mmol per g polar comonomer as used in the definitions in thetext and claims:

The millimoles (mmol) and the micro mole calculations have been done asdescribed below.

For example, if 1 g of the poly(ethylene-co-butylacrylate) polymer,which contains 20 wt % butylacrylate, then this material contains0.20/M_(butylacrylate) (128 g/mol)=1.56×10⁻³ mol. (=1563 micromoles).

The content of polar comonomer units in the polar copolymerC_(polar comonomer) is expressed in mmol/g (copolymer). For example, apolar poly(ethylene-co-butylacrylate) polymer which contains 20 wt. %butyl acrylate comonomer units has a C_(polar comonomer) of 1.56 mmol/g.The used molecular weights are: M_(butylacrylate)=128 g/mole,M_(ethylacrylate)=100 g/mole, M_(methylacrylate)=86 g/mole).

Density

Low density polyethylene (LDPE): The density was measured according toISO 1183-2. The sample preparation was executed according to ISO 1872-2Table 3 Q (compression moulding).Low pressure process polyethylene: Density of the polymer was measuredaccording to ISO 1183/1872-2B.

Method for Determination of the Amount of Double Bonds in the PolymerComposition or in the Polymer A) Quantification of the Amount ofCarbon-Carbon Double Bonds by IR Spectroscopy

Quantitative infrared (IR) spectroscopy was used to quantify the amountof carbon-carbon doubles (C═C). Calibration was achieved by priordetermination of the molar extinction coefficient of the C═C functionalgroups in representative low molecular weight model compounds of knownstructure.

The amount of each of these groups (N) was determined as number ofcarbon-carbon double bonds per thousand total carbon atoms (C═C/1000 C)via:

N=(A×14)/(E×L×D)

were A is the maximum absorbance defined as peak height, E the molarextinction coefficient of the group in question (1·mol⁻¹·mm⁻¹), L thefilm thickness (mm) and D the density of the material (g·cm⁻¹).

The total amount of C═C bonds per thousand total carbon atoms can becalculated through summation of N for the individual C═C containingcomponents.

For polyethylene samples solid-state infrared spectra were recordedusing a FTIR spectrometer (Perkin Elmer 2000) on compression mouldedthin (0.5-1.0 mm) films at a resolution of 4 cm⁻¹ and analysed inabsorption mode.

1) Polymer Compositions Comprising Polyethylene Homopolymers andCopolymers, Except Polyethylene Copolymers with >0.4 wt % PolarComonomer

For polyethylenes three types of C═C containing functional groups werequantified, each with a characteristic absorption and each calibrated toa different model compound resulting in individual extinctioncoefficients:

vinyl (R—CH═CH2) via 910 cm⁻¹ based on 1-decene [dec-1-ene] givingE=13.13 l·mol⁻¹·mm⁻¹

vinylidene (RR′C═CH2) via 888 cm⁻¹ based on 2-methyl-1-heptene[2-methyhept-1-ene] giving E=18.24 l·mol⁻¹·mm⁻¹

trans-vinylene (R—CH═CH—R′) via 965 cm⁻¹ based on trans-4-decene[(E)-dec-4-ene] giving E=15.14 l·mol⁻¹·mm⁻¹

For polyethylene homopolymers or copolymers with <0.4 wt % of polarcomonomer linear baseline correction was applied between approximately980 and 840 cm⁻¹.2) Polymer Compositions Comprising Polyethylene Copolymers with >0.4 wt% Polar Comonomer

For polyethylene copolymers with >0.4 wt % of polar comonomer two typesof C═C containing functional groups were quantified, each with acharacteristic absorption and each calibrated to a different modelcompound resulting in individual extinction coefficients:

vinyl (R—CH═CH2) via 910 cm⁻¹ based on 1-decene [dec-1-ene] givingE=13.13 l·mol⁻¹·mm⁻¹

vinylidene (RR′C═CH2) via 888 cm⁻¹ based on 2-methyl-1-heptene[2-methyl-hept-1-ene] giving E=18.24 l·mol⁻¹·mm⁻¹

EBA:

For poly(ethylene-co-butylacrylate) (EBA) systems linear baselinecorrection was applied between approximately 920 and 870 cm⁻¹.

EMA:

For poly(ethylene-co-methylacrylate) (EMA) systems linear baselinecorrection was applied between approximately 930 and 870 cm¹.

3) Polymer Compositions Comprising Unsaturated Low Molecular WeightMolecules

For systems containing low molecular weight C═C containing speciesdirect calibration using the molar extinction coefficient of the C═Cabsorption in the low molecular weight species itself was undertaken.

B) Quantification of Molar Extinction Coefficients by IR Spectroscopy

The molar extinction coefficients were determined according to theprocedure given in ASTM D3124-98 and ASTM D6248-98. Solution-stateinfrared spectra were recorded using a FTIR spectrometer (Perkin Elmer2000) equipped with a 0.1 mm path length liquid cell at a resolution of4 cm⁻¹.

The molar extinction coefficient (E) was determined as l·mol⁻¹·mm⁻¹ via:

E=A/(C×L)

where A is the maximum absorbance defined as peak height, C theconcentration (mol·l⁻¹) and L the cell thickness (mm).

At least three 0.18 mol·l⁻¹ solutions in carbondisulphide (CS₂) wereused and the mean value of the molar extinction coefficient determined.

DC Conductivity Methods Method A

The plaques are compression moulded from pellets of the test polymercomposition. The final plaques have a thickness of 1 mm and 200×200 mm.

The conductivity measurement can be performed using a test polymercomposition which does not comprise or comprises the optionalcrosslinking agent. In case of no crosslinking agent, the conductivityis measured from a non-crosslinked plaque sample using the belowprocedure. If the test polymer composition comprises the crosslinkingagent, then the crosslinking occurs during the preparation of the plaquesamples, whereby the conductivity is then measured according to thebelow procedure from the resulting crosslinked plaque sample.Crosslinking agent, if present in the polymer composition prior tocrosslinking, is preferably a peroxide, as herein.

The plaques are press-moulded at 130° C. for 12 min while the pressureis gradually increased from 2 to 20 MPa. Thereafter the temperature isincreased and reaches 180° C. after 5 min. The temperature is then keptconstant at 180° C. for 15 min during which the plaque becomes fullycrosslinked by means of the peroxide, if present in the test polymercomposition. Finally the temperature is decreased using the cooling rate15° C./min until room temperature is reached when the pressure isreleased. The plaques are immediately after the pressure release wrappedin metallic foil in order to prevent loss of volatile substances.

If the plaque is to be degassed (i.e. after crosslinking) it is placedin a ventilated oven at atmospheric pressure for 24 h at 70° C.Thereafter the plaque is again wrapped in metallic foil in order toprevent further exchange of volatile substances between the plaque andthe surrounding.

A high voltage source is connected to the upper electrode, to applyvoltage over the test sample. The resulting current through the sampleis measured with an electrometer. The measurement cell is a threeelectrodes system with brass electrodes. The brass electrodes areequipped with heating pipes connected to a heating circulator, tofacilitate measurements at elevated temperature and provide uniformtemperature of the test sample. The diameter of the measurementelectrode is 100 mm. Silicone rubber skirts are placed between the brasselectrode edges and the test sample, to avoid flashovers from the roundedges of the electrodes.

The applied voltage was 30 kV DC meaning a mean electric field of 30kV/mm. The temperature was 70° C. The current through the plaque waslogged throughout the whole experiments lasting for 24 hours. Thecurrent after 24 hours was used to calculate the conductivity of theinsulation.

This method and a schematic picture of the measurement setup for theconductivity measurements has been thoroughly described in a publicationpresented at the Nordic Insulation Symposium 2009 (Nord-IS 09),Gothenburg, Sweden, Jun. 15-17, 2009, page 55-58: Olsson et al,“Experimental determination of DC conductivity for XLPE insulation”.

DC Conductivity Method B (Values in S/Cm) Broadband DielectricSpectroscopy (BDS)

Samples (40×100 mm) were made by hot pressing at 250° C. and 100 kNpress force. Spacers with a thickness of 0.1 mm were used to controlthickness. Disk-shaped samples were then cut out of the plaques.

All measurements were performed on disk-shaped samples with 40 mmdiameter and ˜0.1 mm thickness. The conductivity measurements wereobtained by the use of dielectric spectrometer.

Broadband Dielectric Spectroscopy (BDS) was performed using aNovocontrol alpha spectrometer in a frequency range of 10⁻² to 10⁷ Hz,at different temperatures in the range 253-383K with an error of ±0.1K,at atmospheric pressure and under nitrogen atmosphere. For selectedtemperatures frequency scans were also performed to investigate thelocal and ion dynamics. The sample cell consisted of two silver-coatedelectrodes 40 mm in diameter and the sample with a thickness of about0.1 mm. The complex dielectric permittivity ε*=ε′−iε″, where ε′ is thereal and ε″ is the imaginary part, is generally a function of frequency,ω, temperature T, and pressure P¹, although here only the frequency andtemperature dependencies have been investigated. The complex dielectricconductivity σ* can be also calculated from the complex dielectricfunction ε* as σ*=iωε_(f)ε*, (ε_(f) is the permittivity of free space,8.854 pF/m) where conductivity can also be analysed in a real and animaginary part: σ*=σ′+iσ″. This means the conductivity data areeffectively an alternative representation of the permittivity,nevertheless focusing on different features of the dielectric behaviouras we will discuss below. The analysis has been made using the empiricalequation of Havriliak and Negami²

$\frac{{ɛ^{*}( {\omega,T,P} )} - {ɛ( {T,P} )}}{\Delta \; {ɛ( {T,P} )}} = \frac{1}{\lbrack {1 + {( {i\; \omega \; {\tau_{HN}( {T,P} )}} )a}} \rbrack^{\gamma}}$

where τ_(HN)(T,P) is the characteristic relaxation time in thisequation, Δε(T,P) is the relaxation strength of the process underinvestigation, ε_(∞), is the dielectric permittivity at the limit ofhigh frequencies, and α, γ (0<α, αγ≤1) describe, respectively, thesymmetrical and asymmetrical broadening of the distribution ofrelaxation times. The relaxation times at maximum loss (τ_(max))presented herein have been analytically obtained by fitting therelaxation spectra with the Havriliak-Negami (HN) equation as follows:

$\tau_{\max} = {\tau_{HN}\lbrack \frac{\sin ( \frac{\pi \; \alpha}{2( {1 + \gamma} )} )}{\sin ( \frac{\pi \; \alpha \; \gamma}{2( {1 + \gamma} )} )} \rbrack}^{{- 1}/\alpha}$

Experimental Part

The following materials are used in the examples:

LDPE1—LDPE homopolymer having the properties in table 1:

TABLE 1 Base Resin Properties LDPE1 MFR₂, 190° C. [g/10 min] 0.3 Density[kg/m³] 930 Tensile modulus 350 MPa Flex Modulus 330 MPaLDPE 2—an LDPE copolymer with octadiene of density 922 kg/m³ and MFR₂ of2.0 g/10 min.HDPE1: A bimodal high density polyethylene made using Ziegler Nattacatalysis, density 946 kg/m³ and MFR₂ 0.45 g/10 min.HDPE2: A unimodal high density polyethylene made using Ziegler Nattacatalysis, density 962 kg/m³ and MFR₂ of 12 g/10 min, comonomer butene.

Compounding of the Polymer Compositions:

Each polymer component of the composition was added as separate pelletsto a pilot scale extruder (Prism TSE 24TC), other than the crosslinkingagent. The obtained mixture was meltmixed in conditions given in thebelow table and extruded to pellets in a conventional manner.

Set Values Temperatures [° C.] Extruder Zone Zone Zone Zone Zone ZoneOutput Pressure Filter 1 2 3 4 5 6 rpm [kg/h] [bar] [mesh] 80 155 165175 175 180 225 7.5 60 325

The crosslinking agent if present, was added on to the pellets and theresulting pellets were used for the experimental part.

Example 1—Thermoplastic Blends

TABLE 2 CE1 IE1 CE2 LDPE1 100 98 95 BM ZN HDPE1  2  5 DC Conductivitymethod B (S/cm) 1.10E−14 2.95E−17 3.78E−17

Surprisingly, at 2 wt % of HDPE, conductivity is lower with the bimodalgrade.

Example 2—Crosslinked Polymers

The mixtures in table 3 were prepared as described above.

TABLE 3 Crosslinked Blends Recipe name CE3 IE2 IE3 IE4 CE4 LDPE2 wt %100 99 98 97 90 HDPE2 wt % 0 1 2 3 10 Peroxide (DCP) wt % +0.4 +0.4 +0.4+0.4 +0.4 DC method A (fS/m) 11.1 0.7 0.9 0.6 1.8

1. A cable comprising one or more conductors surrounded by at least aninner semiconductive layer, an insulation layer and an outersemiconductive layer, in that order, wherein the insulation layer is notcrosslinked or is cross-linked and comprises at least 90 wt % of apolymer composition, said polymer composition comprising: (I) 95.5 to99.9 wt % of an LDPE; and (II) 0.1 to 4.5 wt % of an HDPE having adensity of at least 940 kg/m³.
 2. A cable comprising one or moreconductors surrounded by at least an inner semiconductive layer, aninsulation layer and an outer semiconductive layer, in that order,wherein the insulation layer is not cross-linked and comprises at least90 wt % of a polymer composition, said polymer composition comprising:(I) 95.5 to 99.9 wt % of an LDPE; and (II) 0.1 to 4.5 wt % of amultimodal HDPE having a density of at least 940 kg/m³.
 3. The cable ofclaim 1, wherein the cable is a power cable.
 4. The cable of claim 1,wherein the LDPE has an MFR₂ (2.16 kg, 190° C.) of 0.1 to 10 g/10 min.5. The cable of claim 1, wherein the LDPE is a homopolymer.
 6. The cableof claim 1, wherein the HDPE has a density of 945 to 965 kg/m³.
 7. Thecable of claim 1, wherein the LDPE has a density of 920 to 932 kg/m³. 8.The cable of claim 1, wherein its the HDPE has a MFR₂ (2.16 kg, 190° C.)of 0.1-40 g/10 min.
 9. The cable of claim 1, wherein the polymercomposition of the insulation layer has a conductivity of 1.5 fS/m orless when measured according to DC conductivity method as describedunder “Determination Method A”.
 10. The cable of claim 1, wherein thepolymer composition of the insulation layer has a conductivity of3.5×10⁻¹⁷ S/cm or less when measured according to DC conductivity methodas described under “Determination Methods B”.
 11. The cable of claim 1,wherein the polymer composition comprises 1.0 to 3.0 wt % HDPE.
 12. Thecable of claim 1, wherein the insulation layer is crosslinked and theHDPE is unimodal.
 13. The cable of claim 1, wherein the HDPE is a singlesite produced HDPE.
 14. The cable of claim 1, wherein the insulationlayer comprises 99 wt % or more of the polymer composition.
 15. Aprocess for producing a cable comprising: applying on one or moreconductors, an inner semiconductive layer, an insulation layer and anouter semiconductive layer, in that order, wherein the insulation layercomprises at least 90 wt % of a polymer composition as defined in claim1 and is optionally cross-linked.
 16. The cable of claim 3, wherein thepower cable is a direct current (DC) power cable, operating at orcapable of operating at 320 kV or more.
 17. The cable of claim 4,wherein the LDPE has an MFR₂ (2.16 kg, 190° C.) of 0.1 to 5.0 g/10 min.